Process of making pyrolytic graphite-silicon carbide microcomposites

ABSTRACT

A rigid pyrolytic graphite microcomposite material comprising a matrix of pyrolytic graphite containing embedded therein codeposited crystalline silicon carbide comprising aciculae oriented approximately perpendicular to the a-b plane of the crystallite layers of the pyrolytic graphite. The SiC comprises at least about 5 volume percent of the microcomposite material, preferably at least about 10 volume percent. A method for making said microcomposite material comprising pyrolyzing a mixture of methyl trichlorosilane and a hydrocarbon gas at temperatures of about 2800*F to 4000*F, preferably about 3200*F to 3800*F and, thereby codepositing pyrolytic graphite and SiC. A rigid composite pyrolytic graphite article comprising a matrix of the above microcomposite material containing embedded therein at least one reinforcing refractory filament or strand layer. The refractory filament or strand layer comprises a plurality of unidirectional and substantially parallel, laterally spaced, individual, continuous refractory filaments or strands. The microcomposite matrix is nucleated from each of the individual refractory filaments or strands and interconnected to form a continuous matrix phase surrounding and interconnecting the individual filaments or strands comprising the embedded filament or strand layer. A method for making said rigid pyrolytic graphite article comprising winding a continuous, individual, refractory filament or strand around a shaped form and simultaneously pyrolyzing a mixture of methyl trichlorosilane and a hydrocarbon gas onto the filament or strand at about the point of winding contact to nucleate pyrolytic graphite and SiC from the filament or strand, winding additional turns of the filament or strand around the form, each additional turn being spaced from previously wound turns and, as each of the additional turns is wound, simultaneously pyrolyzing the mixture of methyl trichlorosilane and hydrocarbon gas thereon at about the point of winding contact and on the codeposited pyrolytic graphite and SiC nucleated from previously wound turns.

Unite States Patent [191 Olcott 1 Dec. 2, 1975 [75] Inventor: Eugene L.Olcott, Falls Church, Va.

[73] Assignee: Atlantic Research Corporation,

Alexandria, Va.

[22] Filed: Nov. 27, 1972 [211 App]. No.: 309,640

Related US. Application Data [62] Division of Ser. No. 65,899, Aug. 21.,1970, Pat. No.

[52] US. Cl. 427/228; 23/208; 161/59; 161/60; 161/143; 427/249; 427/399[51] Int. Cl B29h 17/28; C230 ll/OO [58] Field of Search 117/46 CA, 46CG, 106,

[56] References Cited UNITED STATES PATENTS 2,789,038 4/1957 Bennett eta1 117/46 CG 3,007,805 11/1961 Cline 106/56 3,158,495 11/1964 Murray etal 117/46 CA 3,317,338 5/1967 Batchelor.... 117/46 CG 3,458,341 7/1969Diefcndorf 117/106 R 3,462,340 8/1969 Hough 161/170 R 3,498,929 3/1970Accountius. 252/503 3,509,017 4/1970 Accountius 23/2094 3,531,249 9/1970Turket 1 106/56 3,580,731 5/1971 Milewski et al 117/107.2 R 3,591,2436/1971 Knippenberg et al. 117/46 CG 3,628,985 12/1971 Hider et a1.117/46 CA 3,629,049 12/1971 Olcott 117/46 CG 3,677,795 7/1972 Bokros eta1. 117/46 CG 3,685,059 8/1972 Bokros et al. 3/1 3,738,906 6/1973 Olcott117/118 3,749,591 7/1973 Hider et a1. 117/46 CA Primary ExaminerWilliamR. Trenor Attorney, Agent, or FirmFinnegan, Henderson, Farabow andGarrett [57] ABSTRACT A rigid pyrolytic graphite microcomposite materialcomprising a matrix of pyrolytic graphite containing embedded thereincodeposited crystalline silicon car'- bide comprising aciculae orientedapproximately perpendicular to the a-b plane of the crystallite layersof the pyrolytic graphite. The SiC comprises at least about 5 volumepercent of the microcomposite material, preferably at least about 10volume percent.

A method for making said microcomposite material comprising pyrolyzing amixture of methyl trichlorosilane and a hydrocarbon gas at temperaturesof about 2800F to 4000F, preferably about 3200F to 3800F and, therebycodepositing pyrolytic graphite and SiC.

A rigid composite pyrolytic graphite article comprising a matrix of theabove microcomposite material containing embedded therein at least onereinforcing refractory filament or strand layer. The refractory filamentor strand layer comprises a plurality of unidirectional andsubstantially parallel, laterally spaced, individual, continuousrefractory filaments or strands. The microcomposite matrix is nucleatedfrom each of the individual refractory filaments or strands andinterconnected to form a continuous matrix phase surrounding andinterconnecting the individual filaments or strands comprising theembedded filament or strand layer.

A method for making said rigid pyrolytic graphite article comprisingwinding a continuous, individual, refractory filament or strand around ashaped form and simultaneously pyrolyzing a mixture of methyltrichlorosilane and a hydrocarbon gas onto the filament or strand atabout the point of winding contact to nucleate pyrolytic graphite andSiC from the filament or strand, winding additional turns of thefilament or strand around the form, each additional turn being spacedfrom previously wound turns and, as each of the additional turns iswound, simultaneously pyrolyzing the mixture of methyl trichlorosilaneand hydrocarbon gas thereon at about the point of winding contact and onthe codeposited pyrolytic graphite and SiC nucleated from previouslywound turns.

6 Claims, 7 Drawing Figures US. Patent Dec. 2, 1975 Sheet 1 Of 23,924,034

US. Patent Dec. 2, 1975 Sheet 2 of2 3,924,034

PROCESS OF MAKING PYROLYTIC GRAPHITE-SILICON CARBIDE MICROCOMPOSITESThis is a division of application Ser. No. 65,899, filed Aug. 21, 1970now US. Pat. No. 3,738,906.

BACKGROUND OF THE INVENTION The superior high temperature and erosionresistant properties of rigid pyrolytic graphite materials are wellknown. These properties make the material particularly useful as linersfor chambers or vessels subject to such conditions, as rocket nozzleinserts, and the like. Pyrolytic graphite, however, does have certaindisadvantageous properties stemming from its particular crystallitestructure and from its tendency to oxidize, particularly at hightemperatures in an oxidizing atmosphere.

Pyrolytic graphite is normally produced by the pyrolysis of acarbonaceous gas, such as methane or propane, onto a heated substrate.Flat, hexagonal crystallites oriented parallel to the substrate surfaceare deposited in layers which build up into an essentially laminarstructure. The pyrolytic graphite crystal is considerably wider in itsflat or a-b plane than along its thickness dimension of c-axis. As aresult, pyrolytic graphite is highly anisotropic in many of itsproperties, including strength, heat conductivity and thermal expansion,with attendant difficulties in practical use. As an example, thematerial has an exceedingly high coefficient of thermal expansion in thethickness or c-axis direction and a relatively low coefficient in thea-b direction. As a result, it is exceedingly difficult to match apyrolytic graphite liner or insert with a suitable backing materialwhich can avoid separation during thermal cycling. Because of itsweakness in the c-direction, due to its flat, plate-like and, thereby,laminar microstructure', pyrolytic graphite tends to delaminate underhigh stresses.

The embedding within the laminar pyrolytic graphite crystallitestructure of aciculae of crystalline SiC which are oriented in thec-direction, as compared to the planar orientation of the layers of thepyrolytic graphite in the a-b direction, advantageously reduces theanisotropy of the graphite and reduces the tendency of the graphite todelaminate. Additionally, it substantially improves oxidation resistancesince, unlike carbon which oxidizes to a gas, silicon oxidizes to Siwhich fuses to form a protective coating. Improved oxidationresistanceis particularly important if the pyrolytic graphite is exposed to hightemperature oxidative atmospheres.

The production of SiC films and coatings, for example, on flexible metalfilaments such as tungsten, by vapor phase pyrolysis of a silane; suchas SiH SiCl SiHCl (CH Si or Ch SiCl with or without added hydrocarbongas, is well known, the objective generally being the production of pureSiC. The pyrolysis temperatures employed are generally below the optimumtemperatures for producing pyrolytic graphite.

Seishi Yajima et al., Journal of Materials Science 4 (1969).pp.t416423and 424-431, and Chemical Abstracts, 1970, 7, p. 69, disclose astructure comprising flake-like single crystals of SiC dispersed in amatrix of pyrolytic graphite and oriented parallel to the planes of thegraphite. The crystallite size of the SiC was about 200 A thick(c-direction) and about 2000 A in diameter (a-b direction). Since thesingle SiC crystals of the Yajima et al. structures are essentially flatand oriented in the same planar direction as the pyrolytic graphite,they cannot have any substantial effect on the anisotropy ordelamination characteristics of the latter.

Yajima et al. pyrolyzed a mixture of SiCl, and propane under vacuum.Maximum SiC production of up to 4 .weight percent was obtained attemperatures of about 1400C to 1500C and dropped to as little as 0.02 to0.03 weight percent at temperatures of about 2000C. Since SiC isconsiderably denser than pyrolytic graphite, the volume percent of SiCwas substantially smaller.

None of the referenced art discloses the pyrolytic graphite-SiCmicrocomposite of this invention or the process for making it.

Copending applications Ser. No. 592,846, now US. Pat. No. 3,129,049, and870,948, now Pat. No. 3,715,253, disclose rigid pyrolytic graphitearticles comprising a matrix of pyrolytic graphite containing embeddedtherein at least one reinforcing layer consisting of a plurality ofunidirectional and substantially parallel, laterally spaced, individual,continuous carbon strands. The matrix comprises crystallite layers ofpyrolytic graphite nucleated from each of the individual carbon strandsand interconnected to form a continuous phase surrounding andinterconnecting the individual strands comprising the embedded strandlayers. By conforming the crystallite pyrolytic graphite layers toembedded strand surfaces instead of to the surface of a conventionalbase substrate, anisotropy of the pyrolytic graphite and its attendantdisadvantages are substantially reduced.

Utilization of the codeposited pyrolytic graphite-SiC microcomposite ofthe present invention in place of the pyrolytic graphite matrixdisclosed in said copending applications provides further improvement inisotropy and improves oxidation resistance.

The object of the invention is to provide a rigid pyrolytic graphite-SiCmicrocomposite having substantially lower anisotropy than pyrolyticgraphite and improved oxidation resistance.

Still another object is to provide a process for making said rigidpyrolytic graphic-SiC microcomposite.

Another object is to provide rigid reinforced composite pyrolyticgraphite-SiC articles having additionally decreased anisotropy.

Still another object is to provide a process for making said rigidreinforced composite pyrolytic graphite-SiC articles.

Other objects and advantages will become apparent from the followingdescription and drawings.

SUMMARY OF THE INVENTION Broadly, the invention comprises rigidmicrocomposite pyrolytic graphite materials containing codeposited andembedded therein crystalline SiC comprising aciculae, the longitudinalaxes of which are oriented approximately perpendicular to the a-b orflat plane of the pyrolytic graphite crystallite layers. Themicrocomposite is a two-phase system since the pyrolytic graphite andSiC are mutually insoluble.

The codeposition of aciculae of SiC within a matrix of pyrolyticgraphite in such manner that the longitudinal axes of the aciculae areoriented approximately in the c-direction relative to the a-b plane ofthe pyrolytic graphite provides a substantial dimension in the thicknessor c-direction which considerably reduces the anisotropy normallycharacteristic of pyrolytic graphite alone. This results insubstantially increased strength in the thickness dimension andimprovement in other properties, such as thermal expansion.Additionally, the perpendicularly embedded SiC aciculae interrupt thelaminar pattern of the pyrolytic graphite and thus reduce its tendencyto delaminate. Since SiC is considerably harder than pyrolytic graphite,the presence of the former in the microcomposite also improveserosion-resistance, as well as the oxidation resistance of the graphite.

The composite pyrolytic graphite-SiC material can be prepared bypyrolyzing a mixture of methyl trichlorosilane and a hydrocarbon gasonto a heated substrate at temperatures of about 2800F, preferably about3200F to 3800F, in a suitable furnace in accordance with proceduresotherwise well known in the production of pyrolytic graphite.

The invention additionally comprises rigid composite articles comprisingthe aforedescribed pyrolytic graphite-SiC microcomposite containingembedded therein at least one reinforcing layer of a plurality ofunidirectional and substantially parallel, laterally spaced, individual,continuous refractory filaments or strands. The pyrolytic graphite-SiCin nucleated from each of the individual refractory filaments or strandsand is interconnected to form a continuous matrix phase surrounding andinterconnecting the individual filaments or strands comprising theembedded filament or strand layer.

Nucleation and growth of the pyrolytic graphite-SiC microcomposite fromthe embedded plurality of refractory filaments or strands furtherreduces and interrupts the laminar character of the pyrolytic graphiteportion of the composite material and thereby further reduces anisotropyand delamination tendency. Additionally, the reinforcing refractoryfilaments or strands increase the strength of the composite article inthe direction of filament or strand orientation.

The rigid reinforced composite pyrolytic graphite- SiC articles can bemade by progressively positioning a continuous, individual refractoryfilament or strand onto a shaped form and simultaneously pyrolyzing amixture of methyl trichlorosilane and a hydrocarbon gas onto thefilament or strand at about the point of positioning contact to nucleatepyrolytic graphite and silicon carbide from the filament or strand,progressively positioning additional filament or strand laterally spacedfrom previously positioned filament or strand and, as the additionalfilament or strand is positioned, simultaneously pyrolyzing the mixtureof methyl trichlorosilane and hydrocarbon gas thereon at about the pointof positioning contact and on the codeposited pyrolytic graphite and SiCnucleated from previously positioned filament or strand. The pyrolysistemperature should be about 2800F to 4000F, preferably about 3200F to3800F.

DRAWINGS FIG. 1 is a photomicrograph at a magnification of 150 of across-section of a sample of the pyrolytic graphite-SiC microcompositeof the invention.

FIG. 2 is a photomicrograph of the same section at a magnification of600.

FIG. 3 is a schematic illustration of apparatus for practicing thisinvention.

FIG. 4 is a schematic illustration of a rigid filamentorstrand-reinforced pyrolytic graphite-SiC composite according to thisinvention.

FIGS. 5 and 6 are schematic representations of modified apparatussuitable for use in preparing the filamentor strand-reinforcedcomposites.

FIG. 7 schematically illustrates an alternative arrangement ofreinforcing strands.

DETAILED DESCRIPTION The amount of SiC should be at least about 5%,preferably at least about 10%, by volume of the microcomposite.Depending upon the desired properties for a particular application, thepercent of SiC can be as high as 90 or even 95. In general, thepreferred range is about 10 to 50 volume percent, with the pyrolyticgraphite making up the remainder.

In some applications, it may be desirable to use a microcomposite ofgraded relative pyrolytic graphite and SiC composition. For example, theoutermost portion of the microcomposite can have a higher SiC content tominimize oxidative surface erosion. Such graded variations in therelative amounts of the codeposited pyrolytic graphite and SiC canreadily be achieved by varying respective flow rates of the methyltrichlorosilane and hydrocarbon gas and/or processing variables in thecodesposition process.

The photomicrographs of FIGS. 1 and 2 at 150X and 600X magnificationrespectively, clearly show the SiC, a large proportion of which is inthe form of needle-like aciculae of SiC oriented substantiallyperpendicularly to the codeposited laminar layers of pyrolytic graphite,which forms an embedding matrix. The volume percent in the photographedsample is about 20%.

The microcomposite can be made by vapor phase pyrolysis of a mixture ofmethyl trichlorosilane and a hydrocarbon gas onto a heated substrate ata temperature of about 28004000F, preferably about 3200-3800F. An inertdiluent gas, such as argon, nitrogen, helium, hydrogen, and mixturesthereof is generally desirable, with some or all of the gas used toaspirate the liquid methyl trichlorosilane. Mixtures of hydrogen withargon, helium or nitrogen has been found particularly effective inobtaining good aciculae crystalline SiC formation. The process can becarried out in a conventional furnace and related equipment at reducedor atmospheric pressures. Atmospheric pressure is generally preferredbecause of the excellent results obtained and the convenience.

The relative flow rates of the methyl trichlorosilane and hydrocargongas vary generally with the desired microcomposite composition. Ingeneral, the silane may be introduced at a weight percent flow rate ofabout 5 to preferably about 15 to 50% and the hydrocarbon gas at aweight percent flow rate of about 25 to 95%, preferably about to 50%.

The hydrocarbon gas can be any of those generally employed in producingpyrolytic graphite by vapor phase deposition, such as the lower alkanes,e.g. methane, ethane, and propane; ethylene; acetylene; and

mixtures thereof. Methane is preferred.

EXAMPLE I A cylindrical graphite substrate was seated in a 4 inchPerenny resistance furnace and heated to 3400F. A mixture of methyltrichlorosilane, methane, argon and hydrogen were injected into one endof the graphite cylinder. The methyl trichlorosilane was entrained forinjection by bubbling argon through a container of the liquid methyltrichlorosilane. Flow rates were: argon l3 std. cu. ft/hr; hydrogen std.cu. ft/hr; methane 2.0 std. cu. ft/hr.

Total methyl trichlorosilane consumed was 85 gm.

Pyrolytic desposition was continued for 1 hour.

The thickness of the formed microcomposite and the relative amounts ofthe codeposited pyrolytic graphite and silicon carbide varied withdistance from the injection nozzle. The thickest portion of themicrocomposite formed was 26 mils and contained about 25 volume percentof needle-like crystalline aciculae of silicon carbide embedded inlaminar layers of pyrolytic graphite. The volume percent of siliconcarbide decreased with increasing distance from the injector. The photomicrographs of FIGS. 1 and 2 were made with a sample taken from adownstream portion having a silicon carbide volume percent of about 20.

The rigid microcomposite cylinder formed by the above procedure wassound and showed no signs of delamination after cooling.

EXAMPLE II A run was made under conditions substantially the same as inExample I except that the pyrolysis temperature was maintained at 3600F.

Results were substantially similar except that at the point of maximumdeposition, the relative volumes of the SiC aciculae and the pyrolyticgraphite were and 85% and then decreased with increasing distance fromthe injector.

The rigid microcomposite cylinder was sound and showed no signs ofdelamination after cooling.

EXAMPLE III A pyrolytic graphite-SiC microcomposite was deposited on a 1inch diameter disc in a manner similar to the procedure used in thepreceding examples except that no hydrogen was used and a one-inch discsubstrate was centered at right angles to the injector so that asubstantially uniform microcomposite was formed over the face of thedisc.

To determine oxidation resistance, the resulting pyrolytic graphite-SiCmicrocomposite disc and a disc of the same size and substrate coatedwith an equal thickness of pyrolytic graphite were heated to about 3000Fin a highly oxidizing oxyacetylene flame for three minutes. Thepyrolytic graphite coating was fully penetrated and almost completelyburned away whereas the pyrolytic graphite-SiC coating eroded only onthe surface with almost half of the thickness remaining intact.

EXAMPLE IV Several pyrolytic graphite and pyrolytic graphite-SiCmicrocomposite deposition runs were made on ATJ graphite discs whichhave a higher coefficient of thermal expansion than pyrolytic graphitein its a-b plane. By cross-sectioning of the deposits, it was determinedthat all of the microcomposites were free from delamination, whereas thepyrolytic graphite deposits showed major delaminations between thedeposit and the substrate.

The pyrolytic graphite-SiC microcomposites can be reinforced to increasestrength and further reduce anisotropy of the pyrolytic graphitecomponent by embedding at least one layer of a plurality ofunidirectional and substantially parallel, laterally spaced, individualcontinuous, refractory filaments or strands in the microcomposite bynucleating the codeposited pyrolytic graphite and SiC from each of thefilaments or strands to form a continuous interconnecting matrixsurrounding and interconnecting the individual filaments or strands.

The strands or filaments can comprise any suitable refractory materialsuch as carbon in any suitable form including, for example, pyrolyzedrayon and pyrolytic graphite; SiC-coateed metal filaments, such astungsten; carbon alloyed with a metal, such as Th, W, Ta, Mb, or Zr, inamounts, for example, up to about 20% by weight; boron filaments, andthe like.

The method can be practiced with apparatus such as that schematicallyillustrated in FIG. 3. As shown therein, a continuous, individualrefractory filament or strand, as for example carbon strand, 1, is fedthrough a guide tube 2, and connected to a mandrel 3, disposed inchamber 4. To prevent oxidation of the carbonaceous gas, atmosphericoxygen is removed and continuously excluded from the chamber byevacuation and/or purging with inert gases such as helium or nitrogen.The strand is heated to and maintained at a temperature sufficient topyrolyze the methyl trichlorosilane and hydrocarbon gases by induction,radiant, or resistance heating means, not shown. The mandrel is rotatedand moved longitudinally relative to the strand guide tube 2, by meansnot shown. In this manner, spaced turns of strand are progressivelypositioned on the mandrel. As the strand is wound, the methyltrichlorosilane, hydrocarbon and carrier gas mixture are fed throughtube 5, to impinge upon the strand at about the point of windingcontact. Pyrolysis of the methyl trichlorosilane and hydrocarbon gasoccurs and a pyrolytic graphite-SiC microcomposite matrix is nucleatedfrom the heated strand substrate. As winding continues, themicrocomposite is simultaneously deposited on the strand being wound andon the matrix deposited on previously wound strands. Thus, the strandsare not only individually enveloped in a microcomposite matrix but areinterconnected and bonded to each other by the matrix. The winding iscontinued to produce a composite article such as schematicallyillustrated in FIG. 4. As shown, the article comprises one or morespaced, reinforcing strand layers 6, each of which comprises a pluralityof spaced strands l, disposed in and interconnected by a pyrolyticgraphite-SiC microcomposite matrix 7, composed of graphite crystallitelayers 8 containing embedded, perpendicularly oriented, codepositedaciculae of SiC.

As shown, the crystallite layers of the pyrolytic graphite in themicrocomposite matrix are oriented in conformity to surfaces of thestrands and are, therefore, aligned around the strands and in thedirection of strand orientation, thereby maximizing strength of thepyrolytic graphite component in that direction. Furthermore, theembedded strands significantly reinforce the microcomposite-strandcomposite in the direction of strand orientation.

Since the orientation of the pyrolytic graphite crystallite layersconforms to the strand surfaces rather than the base or mandrelsubstrate surface of the composite, the pyrolytic graphite component ofthe microcomposite does not have the continuous laminar structurecharacteristic of conventional pyrolytic graphite. This, together withthe embedded codeposited SiC aciculae, further tends to preventpropagation of cracks and delaminations. Composite strength in thethickness direction is also further significantly improved by theincreased degree of crystallite layer alignment in that direction. Inaddition, the marked disparity inthermal expansion in the a-b anddirections characteristic of conventional pyrolytic graphite is furtherreduced.

The strands also prevent delamination failures by restricting thethickness of laminar pyrolytic graphite component growth units nucleatedfrom these strands. It is known that growth units less than 0.05 inchesthick are less subject to delamination. Since, in the composition ofthis invention, the thickness of laminar pyrolytic graphite componentunits is generally about on-half the distance between the strands;preferred unit size is obtained by spacing the strands less than 0.1inch of each other.

The process for composite fabrication can be practiced with individualstrands, as in the embodiment described, or with multi-strandstructures, such as a plurality of laterally spaced, unidirectionallyoriented individual strands, or with woven cloths or tapes comprisingstrands oriented in both warp and woof directions. When usingmulti-strand structures to prepare a composite, it is preferredsimultaneously to impinge the reactive gas mixture on both sides of thestrand structure as it is progressively laid down to ensure that the gaspenetrates between the strands to effect the highest degree of lateralbonding. This can be accomplished by apparatus such as schematicallyillustrated in FIG. 5, wherein gas injector channels 9, feed gas intocontact with spaced strands 1, or by apparatus as shown in F IG. 6,wherein woven refractory cloth 11 and gas are both fed through guidechannel 10.

When the method is practiced with woven fabrics, little matrix bond isobtained between strands where warp and woof intercross since it isdifficult for the reaction gas mixture to penetrate between the touchingstrands. It is, therefore, preferred that all strands in eachreinforcing strand layer in the composite be substantiallyunidirectionally oriented. Such orientation eliminates weaknesses whichresult from the absence of a matrix bond at points of strand to strandcontact. in composites having multiple reinforcing strand layers, thedirection of strand orientation can be varied in different reinforcinglayers as shown, for example, in FIG. 7. Thus composites having desireddirectional strength characteristics can readily be prepared.

This invention can, of course, be practiced by positioning strand on avariety of shaped forms to produce articles having the desiredconfiguration. The strand can be progressively positioned on the shapedform by any desired technique. However, winding is preferred for reasonsof simplicity. It will be understood from the foregoing discussion thatthe term progressively positioning connotes a gradual laying down ofstrand to continuously and progressively increase the area of strandcontact with the shaped form rather than effecting overall lateralstrand contact as by stacking. This permits matrix formation betweenstrands as they are positioned and eliminates the necessity of forcingthe feed gas mixture between prepositioned strands.

When the invention is practiced with strands, such as carbon yarns,which comprise a multiplicity of fibers which have been spun orotherwise incorporated to form the continuous strand, the pyrolyticgraphite-SiC microcomposite may, in some instances be deposited onfibers or fuzz protruding from the strand rather than directly on thebase strand. Therefore, in order to obtain optimum lateral bonding ofstrands by the matrix, it may be desirable to minimize such protrusionsas, for example, by mechanically removing them with a scraper blade asthe matrix is built up or by utilizing strands precoated with pyrolyticgraphite to provide a smooth surface. 4

Although this invention has been described with reference toillustrative embodiments thereof, it will be apparent to those skilledin the art that the principles of this invention can be embodied inother forms but within the scope of the claims.

I claim:

1. A process for making a composite of pyrolytic graphite containingembedded silicon carbide aciculae in which the longitudinal axes of saidaciculae are aligned substantially in the c-direction relative to thea-b plane of the associated pyrolytic graphite crystallite, whichprocess comprises pyrolyzing a mixture of methyl trichlorosilane, aninert gas, and a hydrocarbon gas onto a heated substrate at atemperature of about 2800 F-3800F. and at a pressure at which siliconcarbide aciculae will be formed, the volumetric ratio of inert gas tohydrocarbon gas being at least 86.7 to 13.3 and the relative flow ratesof methyl trichlorosilane and hydrocarbon gas being such that the weightpercent flow rate of methyl trichlorosilane is about 5 to and the weightpercent flow rate of hydrocarbon gas is about 25 to 2. The process ofclaim 1 in which the temperature is about 3200F3800F.

3. The process of claim 1 in which the hydrocarbon gas is methane.

4. The process of claim 1 in which the inert gas is a mixture ofhydrogen and argon, helium, or nitrogen.

5. The process of claim 1 in which the methyl trichlorosilane is presentin amount sufficient to provide at least about 5% by volume of siliconcarbide in said microcomposite.

6. The process of claim 1 in which the pyrolysis is carried out atatmospheric pressure.

1. A process for making a composite of pyrolytic graphite containingembedded silicon carbide aciculae in which the longitudinal axes of saidaciculae are aligned substantially in the c-direction relative to thea-b plane of the associated pyrolytic graphite crystallite, whichprocess comprises pyrolyzing a mixture of methyl trichlorosilane, aninert gas, and a hydrocarbon gas onto a heated substrate at atemperature of about 2800* F-3800*F. and at a pressure at which siliconcarbide aciculae will be formed, the volumetric ratio of inert gas tohydrocarbon gas being at least 86.7 to 13.3 and the relative flow ratesof methyl trichlorosilane and hydrocarbon gas being such that the weightpercent flow rate of methyl trichlorosilane is about 5 to 75% and theweight percent flow rate of hydrocarbon gas is about 25 to 95%.
 2. Theprocess of claim 1 in which the temperature is about 3200*F-3800*F. 3.THE PROCESS OF CLAIM 1 IN WHICH THE HYDROCARBON GAS IS METHANE.
 4. Theprocess of claim 1 in which the inert gas is a mixture of hydrogen andargon, helium, or nitrogen.
 5. The process of claim 1 in which themethyl trichlorosilane is present in amount sufficient to provide atleast about 5% by volume of silicon carbide in said microcomposite. 6.The process of claim 1 in which the pyrolysis is carried out atatmospheric pressure.